In environments where ambient noise is present, it is often difficult for users of mobile telephones or other audio devices to hear desired audio signals. In order to overcome this, noise cancellation may be carried out to cancel the ambient noise in the user's ear. In a noise cancellation system, a microphone is provided to detect ambient noise as it approaches the user's ear. The ambient noise signal that is generated by the microphone is then processed to form a noise cancellation signal. This noise cancellation signal is then applied to a speaker that is preferably located close to the user's ear. The signal processing is intended to be such that the sound that is generated by the speaker in response to the noise cancellation signal is of equal magnitude but exactly opposite phase to the ambient noise signal, as the two sounds reach the user's ear. If this can be achieved, noise cancellation occurs.
The gain that is applied as part of the signal processing to form the noise cancellation signal may be controlled, so that the degree of noise cancellation can be adjusted.